Deformation sensors based on radio-frequency resonant cavities whose resonant frequency depends on the size of the resonant cavity are well known. Such sensors are inserted in the stressed structural material to be monitored. When the material is subjected to a stress such as to alter the size of resonant cavity, the resonance frequency changes. Therefore, by processing the resonant frequency changes it is possible to trace the deformation of the structure. The sensor is typically equipped with a radio-frequency communication device, for example a RFID tag (“RFID tag”) (Radio Frequency IDentification), to receive a variable-frequency radio signal from a remote interrogator device, to transfer such a radio signal to the resonant cavity for exciting it and to transmit back the cavity reaction to the interrogator device. In this way it is possible to remotely interrogate the sensor without physical connection, that is “wirelessly”.
The resonant-cavity sensors provide fairly reliable measurements, but have the disadvantage of being rather bulky, and therefore of being particularly invasive and difficult to be installed.
An alternative is provided by the piezoelectric sensors based on MEMS technology, which sensors convert the mechanical variation of the structures to which they are attached in electric signal changes. These sensors have the advantage of having reduced dimensions, compared to those based on resonant cavities, however at the expense of lower reliability. This type of sensor too is provided with a radio-frequency communication device to transmit the information coming from the sensor to the interrogator device.
Other types of sensors are based on optical fiber Bragg gratings. Such sensors provide very accurate deformation measurements but are extremely expensive and bulky as well.